This invention relates to a process of preparing benzaldehyde or a substitute expressed by C.sub.6 H.sub.(5-n) X.sub.n CHO, wherein X represents a halogen atom or trifluoromethyl group and n is 1 or 2, by vapor phase catalytic reaction between water and benzal chloride or a substitute expressed by C.sub.6 H.sub.(5-n) X.sub.n CHCl.sub.2.
Benzaldehyde and its substitutes of the above defined class are industrially of use as materials for the synthesis of various organic compounds including some medicines and agricultural chemicals. It is possible to obtain benzaldehyde or its substitute of the above defined class by hydrolysis of benzal chloride or its substitute of the above defined class, but it is impracticable to achieve the hydrolysis by merely heating a mixture of benzal chloride or its substitute with water because the hydrolysis proceeds only at a very low rate.
Accordingly various catalysts have been proposed for liquid phase hydrolysis of benzal chloride or its substitute to form benzaldehyde or its substitute. Typical examples of the hitherto proposed catalysts are as follows: (1) aqueous solution of acid or alkali; (2) cuprous chloride or cupric chloride; (3) aqueous solution of iron salt; (4) anhydrous zinc chloride; and (5) zinc oxide.
However, every process using one of these catalysts is disadvantageous in certain respects from an industrial point of view. More particularly, the use of an aqueous solution of an acid or an alkali (1) is liable to cause undesirable side reactions and, besides, is almost ineffective for the hydrolysis of substituted benzal chlorides having an electron attractive group typified by trifluoromethyl group. Furthermore, if this process is put into industrial practice it becomes a requisite to the process to use a reaction vessel of very large capacity relative to the quantity of the compound subjected to hydrolysis, and the waste acid or alkali must be treated with considerable trouble. Any one of the processes using the metal salt catalysts (2), (3) and (4) suffers from an unsatisfactorily low rate of reaction, and this problem becomes very serious when the starting material is a substituted benzal chloride having an electron attractive group, and an increase in the quantity of the metal salt catalyst with a view to enhancing the reaction rate significantly promotes unfavorable side-reactions. The use of zinc oxide catalyst (5) is almost ineffective for hydrolysis of a trifluoromethyl-substituted benzal chloride and, besides, is unsuitable to a continuous process because of the need for the step of separating zinc oxide from the reaction product.
Due to such problems or disadvantages, none of the hitherto proposed liquid phase catalytic hydrolysis processes can be taken as suitable to industrial practice.
Japanese Patent Application Primary Publication No. 48(1973)-5733 proposes a vapor phase catalytic reaction process for the hydrolysis of benzal chloride or its substitute characterized by using silica or alumina as catalyst either in pure state or in a state impregnated with cuprous chloride or cupric chloride. This process can be performed as a continuous process since the reaction takes place in vapor phase, but this process has the following shortcomings. In the case of using either silica or alumina in pure form as the catalyst, it is practically impossible to achieve the intended hydrolysis of a trifluoromethyl-substituted benzal chloride firstly because the rate of the reaction is very low even at an initial stage where the catalyst is in a fresh state, and secondly because the catalyst is easily fluorinated by hydrogen fluoride formed by hydrolysis of a portion of the trifluoromethyl group of the starting material and, therefore, is rapidly deactivated. Even in the case of using a silica or alumina catalyst impregnated with cuprous chloride or cupric chloride, the activity of the catalyst is insufficient for efficient hydrolysis of a trifluoromethyl-substituted benzal chloride and rapidly and significantly lowers with the lapse of time, so that the hydrolysis can hardly be performed as a truly continuous process.